Vacuum distillation of petroleum hydrocarbons is a well known refining process commonly utilized in the art to minimize thermal cracking of heavier fractions of crude oil and obtain lighter desired products. Distilling these heavier materials under vacuum, that is lower pressure, decreases the boiling temperature of the various hydrocarbon fractions in the feed and therefore minimizes thermal cracking of these fractions. In conventional vacuum distillation systems, distillation is carried out in a vacuum column under pressures typically in the range of 25 to 100 millimeters of mercury (mmHg). It is important in such systems to reduce pressure as much as possible to improve vaporization. Vaporization is enhanced by various methods such as the addition of steam at the furnace inlet and at the bottom of the vacuum distillation column. Vacuum is created and maintained using cooling water condensers and steam driven ejectors. The size and number of ejectors and condensers used is determined by the vacuum needed and the quantity and quality of vapors handled. Three ejector stages are usually required for a distillation column flash zone pressure of 25 mmHg. In a conventional system of this type, the first stage condenses the steam and compresses non-compressible gases, while the second and third stages remove the non-condensable gases from the condensers. The vacuum produced is limited to the vapor pressure of the water used in the condensers. If colder water is supplied to condensers, a lower absolute pressure can be obtained in the vacuum tower.
In contemporary conventional vacuum distillation systems, the most common design includes a condenser upstream of the ejectors, known as a pre-condenser, to reduce the size and steam consumption of the vacuum system. FIG. 1 illustrates a pre-condenser type vacuum system. In this type of system, the lowest achievable pressure is the partial pressure of the process vapor in the pre-condenser, which is determined by the temperature of the condensing fluid, usually water. In a pre-condenser type system, the process vapor from the pre-condenser is usually 80 to 90 percent water so the lowest pressure achievable is about 15 percent greater than the vapor pressure of water at the equilibrium vapor outlet temperature, which as noted is equal to the temperature of the condensing liquid, usually water. In warm, humid climates, such as the US Gulf Coast where the average wet bulb temperature in the summertime is 80° F., the lowest achievable pressure in a pre-condenser type vacuum system is about 50 mmHg when condenser-cooling water is available only at ambient temperatures. Adding more ejectors or increasing the capacity of the existing ejectors in the system does not result in lower distillation column pressure because of the high percentage of water in the process vapor.
To further minimize thermal cracking, it is desirable to achieve pressures in vacuum distillation columns lower than the 40 to 60 mmHg typical of pre-condenser type vacuum systems. This problem is addressed in vacuum distillation system designs known as “deep-cut technology”, which involves installation of primary ejectors upstream of any condensers in the process flow scheme. FIG. 2 illustrates a “deep-cut” type vacuum distillation system. In this type of system, the vacuum distillation column overhead vapor is compressed by the primary ejector which allows the column to operate at low absolute pressures in the range of 5 to 20 mmHg. As illustrated in FIG. 2, the large primary ejector discharges first into a large condenser, known as a first inter-condenser. The first inter-condenser must condense not only the column stripping and furnace lift steam, but also the motive steam used to power the primary ejector.
In deep-cut type vacuum distillation systems, the primary ejectors are designed for a specific suction load and discharge pressure, also known as backpressure. The process or suction load to the primary ejector is determined by the vacuum column process design requirements (principally process steam injection rate and desired vacuum gas oil make) while the backpressure is determined primarily by the design of the first inter-condenser. In the art, the normal practice is to design the ejectors to operate under worst case conditions; that is, when the cooling water available is the warmest and the heat exchanger in the condensing system is fouled. Thus, when cooling water temperature is colder than such “worse case” design conditions, such as in the cool season months in sub-tropical regions, extra condensing capacity is available and the first inter-condenser. This extra condensing capacity results in a lower than design backpressure in the primary ejector.
In sub-tropical regions of the Northern hemisphere, such as the US Gulf Coast, the average temperature of available cooling water during the year varies between about 60° F. and 85° F. During the cool season months, lower cooling water temperatures result in a pressure in the first inter-condenser can be expected to range between about 35 mmHg and 60 mmHg, whereas during the warm season months, the expected absolute pressures in the first inter-condenser typically are about 20 percent to 40 percent higher. Pressure in the first inter-condenser represents backpressure on the primary ejector, which, as noted, is designed to operate efficiently under the worst-case conditions prevalent during the warm season months. Thus, the primary ejector is not typically designed to take advantage of lower backpressures available as a result of lower temperature cooling waters available during the cooler season months.
FIG. 4 depicts a characteristic performance curve for a primary ejector and illustrates and provides an example of the problem to be solved by the present invention. Thus, as illustrated in FIG. 4, as the process or suction load to the primary ejector increases, so does the suction pressure and therefore the column pressure. In the example shown in FIG. 4, the primary ejector is designed for a process load in the range of 8,500 to 10,500 pounds per hour to achieve a suction pressure in the range of a 13 to a 17 mmHg. This standard performance curve does not show the effect of backpressure from the first inter-condenser on the ejector. Under this circumstance, lowering the backpressure will not have any measurable effect on the ejectors suction pressure. Thus, the extra condensing capacity available from lower temperature cooling water available during the cool season months does not result in a lower column pressure. Available methods to reduce the column pressure, such as decreasing the suction load by decreasing the column stripping steam injection rate or reducing the furnace lift steam injection rate, has the effect of decreasing efficiency of the vacuum column and reduces the recovery of desirable gas oil product. Another method to reduce the vacuum column pressure would be to install larger primary ejectors; however, larger primary ejectors requires installation of facilities for injection of more steam, installation of larger condensers and systems for circulating larger amounts of cooling water. Installation of such systems would, of course, involve undesirable capital expenditure. Accordingly, the object of the present invention is to take advantage of seasonal variations in cooling water temperatures, by installing an additional ejector that can be seasonally operated in parallel with the primary ejector that has the effect of reducing the process or suction load to the primary ejector and therefore reduces the vacuum column pressure. See FIG. 4.